The invention relates to the scanning and reproduction of originals bearing pictorial images, and the like.
In many photographic copying processes -- for example, the production of color-paper images, the duplication of diapositives, the production of color negatives from color positives or the making of black-and-white take-offs from color films -- it is necessary or desired to change the gradation of the reproduced image, compared to the gradation of the image on the original. In general what is involved, in effect, is a reduction of the value of the exponent gamma of the characteristic curve. Dark regions should be brightened, and light regions darkened; however, other gradation changes may also be desired in some circumstances.
It is known to use as the light source in such a copying operation a cathode ray tube the spot brightness of which is controlled. The flying spot formed on the image screen of the tube traces a raster, which is projected onto a transparent original. The major portion of the transmitted light is projected by means of image-forming optics onto the light-sensitive material, causing an image-forming chemical reaction. The remainder of the transmitted light is directed onto a photodetector which generates a signal for controlling the intensity of the illuminating flying light beam derived from the flying spot of the cathode ray tube. In this way, the spatial distribution of the density of the transparent image has a controlling effect upon the illuminating light source; this feedback action results in a change of the contrast of the reproduced image compared to the contrast of the image on the original.
With the known system, in consequence of the aforedescribed feedback, there are formed in the reproduced image a plurality of dark and bright stripes or striations oriented parallel to an edge which runs perpendicular to the scanning direction. The width of these stripes depends upon the propagation-time-dependent phase shift introduced in the electro-optical feedback branch of the system.